The answer to this question is 'yes'. You are allowed to port to a third company while your petition with the second company is still being processed provided you are able to document clearly that you have always been employed and in status with each of the prior companies.

Yes, that should be possible. However, the Employer must be able to document the change of employment location by filing a Labor Condition Application (LCA) and even an amended petition if the situation warrants. An amended petition is required where there has been a "material change" in the nature of employment. This change could affect [...]

In this case, the Beneficiary will be able to travel and reenter the U.S. in F status as long as he has a valid F visa and the H-1B petition (with an October 1st start date) is approved. The Change of Status (from F to H) petition would be considered abandoned if the Beneficiary travels [...]

Using the EAD will not impact your Husband's H-1B status. In fact, you are right in planning to have your husband retain his H-1B status (as you put it, "in case something were to happen to the I-485.").

The transfer is possible. A new Petition would have to be filed with the receipt of the pending H petition. If this is a CAP subject petition, no additional proof is necessary to file a second petition with a different employer.

From your email below it appears you currently have a valid H-1B approval. However, this approval does not correspond to the visa on your passport. Well, visas are not employer specific and therefore, you could travel and re enter with the valid visa and your current H-1B approval. A visit to the Embassy would not [...]

Yes, you may go back to work for your Employer in this situation. H-1B approvals remain until revoked.

You could wait, but know that it is not always that the USCIS issues an RFE in this situation. They could outright deny your I-140 and I-485. They could also invalidate the EAD and you could find yourself in a serious situation without proper work authorization. Proactively complying with AC21 portability is recommended.

Your permanent resident husband could always file an immigrant petition on your behalf. You would become eligible to apply for Adjustment of Status as soon as your 'priority date' becomes current. Additionally, you should be able to reclaim any time that you have spent abroad towards your H-1B stay in the U.S. If you decide [...]

EAD adjudication has been delayed due to significant changes to the procedure involved in accepting and processing these applications. Contractor errors contributed to significant delays as well. As such, you could always call to expedite your EAD if you can show that you are likely to lose your job, or the Employer would suffer serious [...]